In disk-type magnetic recording systems for digital applications, magnetic transducer elements, or heads, are used to record information onto (i.e., write) or retrieve information from (i.e., read) the disk surface or surfaces. Each storage disk comprises an annular substrate onto which is deposited a magnetic recording medium. Each disk surface is divided into several concentric, annular bands, or "tracks" each having a predetermined radial extent. Adjacent tracks are separated by an unused buffer zone. Each head is supported in close proximity to an associated disk surface by a head positioning assembly, or actuator, that supports the head near the disk surface and moves it from one radial position to another, thereby permitting use of a single head for reading and writing on multiple tracks. The positioner assembly for each head or group of heads includes an actuator arm and an actuator motor. The actuator motor moves the actuator arm, to change the position of the head with relation to the tracks on the disk. A disk drive may include a plurality of stacked disks, and one actuator motor may be used to move a corresponding number of actuator arms in unison.
Head positioning assemblies are generally of two types: (1) linear and (2) rotary. Linear positioners move the actuator arms and heads along a substantially linear path oriented along or parallel to a radius of the recording disk(s). Rotary positioners, by contrast, rotate the actuator arm(s) about a pivot point outside, but close to, the rim of the recording disk(s). This invention relates to rotary head positioners.
Reliable high speed disk memory operation requires, first, that the position of the read/write head relative to a track on the disk be maintained within extremely close tolerances; and, second, that the access time (that is, the time required to move the head from one track to another desired track) be short. The state of the art concerning the first requirement necessitates that a control system, preferably utilizing feedback, be employed to sense the deviation of the position of the head from an optimum read/write position over the track, and to generate a correction signal for driving the actuator motor. A short access time, on the other hand, requires that the moving mass and inertia of the positioner and heads be kept as small as possible. Reducing the mass leads to other problems, however, including twisting and bending of the arms as they are pivoted back and forth over the disk surface. This can lead to vibrations in the arm at or near certain resonant frequencies which are pumped by the positioner's servo system. If the resonant frequencies of the arms are too low, the vibrations can cause large amplitude motions to be excited which, in turn, leads to the possibility that a head can crash into the disk or vibrate so far from its intended position as to be out of range for properly reading or writing the track. To eliminate these problems, the stiffness of the arm may be increased, thereby raising the resonant frequencies of the structure and substantially reducing the amplitude of motions induced by any vibrations that may occur at lower frequencies or by servo pumping.
With a linear positioner, the head support arm is accelerated in a straight line over the disk, usually in a radial direction. This direction of motion is also the direction of the longitudinal axis of the arm itself; hence, the arm exhibits great stiffness. By contrast, a rotary positioner accelerates the arm in a lateral direction, subjecting it to considerable bending and perhaps torsional moments. For a typical arm, bending and torsional stiffness are considerably less than axial stiffness. However, rotary positioners offer many performance advantages not found in linear positioners; hence, if a rotary arm can be made sufficiently stiff to resist bending and twisting, while still maintaining a low mass and inertia, then a superior product results.
Rotary positioners of the prior art typically consist of several arms, in spaced apart relationship, stacked one above the other, pivoted at their centers on a common pivot, with read/write heads mounted at one end and the moving coil(s) of the rotor of the actuator motor mounted at the other. The stator portion of the motor includes the permanent magnets for the actuator motor. The coil thus also acts as a counterweight to balance the heads. This configuration, however, suffers from a lack of stiffness and from excessive mass and a large moment of inertia; as such, it normally is used only on low or medium performance disk memories.